Electron Density Distribution in HSX

1
Evidence for Fast-Electron-Driven Alfvénic Modes
in the HSX Stellarator

• D.L. Brower and C. Deng
• University of California, Los Angeles
• D.A. Spong
• Oak Ridge National Laboratory
• A. Abdou, A.F. Almagri, D.T. Anderson, F.S.B.
  Anderson, S.P. Gerhardt,
• W. Guttenfelder, K. Likin, S. Oh, V. Sakaguchi,
  J.N. Talmadge, K. Zhai
• University of Wisconsin-Madison

June 28, 2005 EPS-Tarragona
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HSX Provides Access to Configurations With and
Without Symmetry QHS helical axis of symmetry
in B predicted very low neoclassical transport
Mirror quasi-helical symmetry broken by adding a
mirror field.
QHS
Mirror
Red?B?0.5 T Blue?Blt0.5 T
Mirror Helical Bands are Broken
QHSHelical Bands of Constant B
helical axis of symmetry, no toroidal curvature,
no toroidal ripple
Conventional stellarators exhibit poor
neoclassical transport in low-collisionality
regime due to magnetic field ripple
3
HSX major radius 1.2 m minor radius 0.15
m magnetic field 0.5 T 28 GHz ECRH lt150
kW pulse length lt 50 ms
4
Outline

• Characteristics of observed fluctuations
• Quasi-Helically Symmetric (QHS) configuration
• Mirror (MM) configuration (conventional
  stellarator)
• Alfvén Continua for QHS and Mirror Mode Plasmas
  (conventional stellarator) in HSX
• Evidence for fast-electron driven GAE mode
• Effect of biasing on Alfvenic mode

• GOAL
• Observe Alfvénic modes driven by fast electrons
• Quasi-Helical Symmetry makes a difference

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Flux Surfaces and Interferometer Chords

• Interferometer System
• 9 chords
• 200 kHz B.W.
• 3. 1.5 cm chord spacing

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Coherent Density Fluctuations
QHS plasma
fluctuation
noise
- 28 GHz ECRH - 2nd Harmonic X-mode - Generates
fast electrons with
For PECRH gt 100 kW, confinement degrades Mode
perturbs particle orbits leading to enhanced loss
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No mode observed in Mirror Configuration Plasma
QHS
mirror
10 Mirror perturbation
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Fluctuation Features

• only observed in QHS plasmas
• coherent, m1 (n?)
• localized to steep gradient region
• satellite mode appears at low densities, Df20
  kHz
• Electromagnetic component

modd 1?
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Observed Fluctuations Associated with ECRH
- Mode disappears 0.2 msec after ECRH turn-off,
- faster timescale than WE and soft x-rays - 2nd
Harmonic X-mode generates nonthermal electrons
(ECE) (no source for fast ions Ti20 eV)
Modes driven by energetic electrons?
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Alfvénic Modes

• Historically, Alfvénic modes have been observed
  on tokamaks or stellarators with NBI or ICRF to
  generate fast particles.
• Alfvénic modes are generated if
• resonance condition (Vp
  particle velocity)
• for trapped particles,
• where wDh is the trapped-particle precessional
  drift frequency,
• depends on particle energy, not mass
• unstable when wdia gt wAlfven
• where wdia is the diamagnetic drift frequency
• energetic ions or electrons can drive
  instability

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HSX Quasi-Helically Symmetric (QHS)
configuration Normal mode Alfvén continuum n
1 mode family

• GAE Gap B0.5 T
• 0 - 50 kHz for m1,n1 ne(0)1.8x1012 cm-3
• Only minor changes for mirror configuration

GAE
B0.5 T
3-D STELLGAP code (D. Spong)
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Mode frequency scaling with ion mass density
What about B scaling? Future 1 T operation
- frequency and mass density scaling consistent
with Alfvenic mode - If iota is lowered lt 1, GAE
gap disappears and mode not observed
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Density fluctuations decrease with introduction
of symmetry breaking (toroidal mirror) term
Fluctuation no longer observed for Mirror
perturbation gt2 (conventional stellarator
configuration 10 mirror perturbation)
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Soft X-ray, Hard X-ray Emission for QHS and Mirror

• Soft X-ray (600 eV-6 keV) emission
• QHSgtgtMirror
• Hard X-ray flux
• QHSgtgtMirror
• decay time longer
• fast particles are better confined in QHS
• .wDhwGAE
• 5-10 keV particles

QHS
Mirror
- fast particles (trapped electrons) are better
confined for QHS - provide drive for Alfvenic
modes
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Result QHS Flows Damp More Slowly,
and, Go Faster For Less DriveViscous
Damping is Reduced for QHS
QHS 8 A of electrode current
QHS
Mirror 10 A of electrode current
Mirror
other parameters (ne1x1012cm-3, nn ? 1x1010cm-3
Ti?25eV, B0.5T, PECH50 kW) held constant.
S.P. Gerhardt et al., PRL 94,015002(2005)
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QHS biasing increases amplitude and decreases
frequency
nedl
Bias

• - amplitude
• increases 50-100
• frequency
• decreases 10-20

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Alfvenic mode frequency shift can be used to
measure core flow dynamics
During biasing ne and B do not change so
VA is constant Ambient plasma potential is
() ExB flow in ion drift direction Alfvenic
mode propagates in electron diamagnetic drift
direction?
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QHS - biasing decreases mode amplitude and
increases frequency
nedl
Bias
- Biasing against direction of ambient flow
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Mirror Mode Alfvenic Mode observed with biasing
nedl
Bias
No Alfvenic mode observed between bias pulses
Mirror Mode mode only observed w/bias in
direction of ambient flows
Er acting to reduce neoclassical losses?
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Evidence for fast-electron-driven Alfvenic modes
in HSX

• Calculations of Alfven Wave Continuum by 3-D
  STELLGAP code shows the possibility of GAE mode
  in HSX
• Measure a coherent fluctuation global mode modd
  (1?) with frequency and ion mass density scaling
  is consistent with Alfvénic mode (B scaling
  unknown).
• Measurements suggest that the fluctuation is most
  likely driven by non-thermal electrons
• Alfvenic Mode is only observed for QHS
  configuration, not for Mirror Configuration (2)
  improved (trapped) particle confinement for QHS
• Biasing Dflab may provide information on core Er
  and flow dynamics!
• - How do flows affect to Alfvenic mode growth
  rate?

Mode amplitude can be controlled by (1) flows and
(2) configuration
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Open Issues

• Mode propagation direction edd if Er becomes
  more () with biasing. Expect mode to propagate
  in diamagnetic drift direction of driving species
• Mode structure modd (1?), n? External
  magnetics suggest m0? Differences between
  magnetic and density measurements..
• B scaling? need to know Er profile. Can
  frequency be explained by plasma rotation?
• Source of satellite frequencies (1) different
  m,n, or (2) different roots of the same MHD
  equations (different radial structure with same
  m,n?
• Sensitivity of Alfvenic mode to mirror
  perturbation (2). Which particles are resonant
  with mode? How are they affected by mirror
  perturbation?
• Biasing Dflab may provide information on core Er
  and flow dynamics!
• - How do flows affect to Alfvenic mode growth
  rate?
• - Er measurements in plasma core How is
  potential profile modified by biasing? What is
  potential profile for non-biased plasmas?

Investigation of Alfvenic modes in HSX has just
begun!
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Mean ?float Profiles Change Significantly with
Density

• Similar edge Isat profiles
• Drastic change in floating potential
• Inferred Er changes from positive to negative
  inside r/a ? 1.0
• Does not take into account Te profile

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QHS and Mirror Mode Density Profilesne 1x1012
cm-3
Chord position
QHS

• Magnetic perturbation
• shifts flux surfaces
• particles tied to field lines
• interferometer chord fixed
• generates density fluctuation

Mirror Mode
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HSX Mirror Mode (MM) configuration Normal mode
Alfvén continua n 1 mode family
Mirror Mode toroidal mirror term introduced to
magnetic configuration,
equivalent to conventional stellarator operation

• GAE Gap B0.5 T
• 0-60 kHz, m1, n1
• ne(0) 1.8x1012 cm-3

B0.5 T
GAE
10 mirror perturbation (STELLGAP code)
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QHS biasing increases Alfvenic Mode amplitude
During () biasing (1) ne and B do not change
so VA (fGAE) is constant (2) flab is
reduced, so GAE must be going opposite to
ExB (3) GAE mode propagates in electron
drift direction
Er

electron diamagnetic drift direction
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Coherent Density Fluctuation Mode
Frequency chirping sometimes observed (implies
nonlinear interactions)